The present invention relates to plastic injection molding and more particularly to plastic injection molding using gas assist and the formation of hollow rib members on plastic injection molded parts.
In the plastic injection molding art, the usual challenges facing the product designer include designing an article having the requisite strength for the product application and satisfactory surface finish, as well as avoiding excessive weight, surface distortions, and increased cycle time. For flat or thin products, it is typical to include one or more rib members in the design to provide relative strength and structure for the molded article. The rib members are typically thicker than the molded article which increases the weight, material usage, and cycle time of the article, and often induces sink marks and other surface defects due to a thermal gradients in the area of the thickened section.
It is known in the-plastic molding art to use pressurized gas, such as nitrogen, in conjunction with plastic injection molding of articles. Pressurized gas serves several purposes. The gas allows the article or rib structure to have hollow interior portions which result in savings in weight and material, thereby reducing costs. The pressurized gas also applies an outward pressure to force the plastic against the mold surfaces while the article solidifies. This helps provide a better surface on the molded article and also reduces or eliminates sink marks and other surface defects. The use of pressurized gas also reduces the cycle time as the gas is introduced and/or migrates to the most fluent inner volume of the plastic and replaces the plastic in those areas which would otherwise require an extended cooling cycle. The pressure of the gas pushing the plastic against the mold surfaces further increases the cooling effect of the mold on the part, thus solidifying the part in a faster manner and reducing the overall cycle time.
Where the rib members or other portions of the article in which the gas is being introduced are elongated, it is often difficult to provide a satisfactory molded article. For example, if the pressure of the gas is too great as it enters the mold cavity, there is a risk that it may rupture or blow out the plastic within the mold cavity, i.e. the gas is not contained within the plastic. Also, it is often difficult to have the gas migrate along the full length of an elongated, thicker plastic section, thus creating a product which has an uneven thickness and cooling cycle.
One manner which has been developed in order to overcome some of the above-mentioned problems is shown in U.S. Pat. No. 5,098,637. In that process, a secondary cavity (a/k/a xe2x80x9cspilloverxe2x80x9d cavity) is provided at one end of the molded part or elongated rib member in order to collect and contain the more fluent plastic material which is forced out of the article or rib member by the pressurized gas. Another system is shown in U.S. Pat. No. 5,885,518.
An object of the present invention is to provide an improved method of gas assisted injection molding. It is another object of the present invention to provide an improved method and system for injection molding plastic parts with structure rib members.
It is a still further object of the present invention to provide a method and system for gas assist injection molding which eliminates the need for spillover cavities and the subsequent capture and regrinding of excess plastic material from a mold cavity. It is an additional object of the present invention to provide a moveable insert in a mold cavity and allow packing of the plastic material in the mold for better dimensional and surface effects.
In accordance with the objects of the present invention, one or more movable insert members are provided in the mold cavity. The insert members are particularly located in portions of elongated rib members in which pressurized gas is to be introduced in order to provide hollow portions in them. The insert members can be held in place by spring or other biasing members or mechanisms which are adapted to be overcome by the force of the plastic and pressurized gas when the gas is introduced into the rib members.
A full-shot of plastic material is first injected into the mold cavity. The plastic material is then packed in the mold by additional pressure plastic injection from the molding machine. Thereafter, pressurized gas is introduced into at least one of the rib members. As the gas channel is formed in the rib member(s) and the fluent plastic material in the center of the rib member(s) is pushed along the rib member(s) and out the opposite end(s), the spring or biasing force on the insert member(s) in one embodiment of the invention is overcome. As the insert member(s) retracts, the plastic material fills the entire article defining mold cavity. The displaced plastic material flows into the area formerly occupied by the insert member(s), creating an additional rib or part of the molded article.
Once the entire article is formed, the plastic is allowed to cool and solidify. The injected gas pressure is then relieved (vented or exhausted) from the formed article and the article is ejected or removed from the mold.
In one preferred embodiment, the insert member is held in its initial position by spring tension, pressurized cylinders, damping mechanisms, or the like. These tensioning or biasing mechanisms provide a sufficient biasing force to resist the level of force caused by pressure in the mold from the initial injection of the plastic material. This maintains the nominal wall thickness of the article being molded throughout the full extent of the mold with the exception of the thicker section of the rib members. Once the cavity is full, pressurized gas is introduced at the end of the rib members opposite the insert members. The force of the gas pressure and the resin forced from the rib members overcomes the biasing force of the insert members.
In order to withstand the additional packing pressure applied to the plastic material in the mold, the insert members are fixed or locked into position by use of a releasable locking mechanism. The locking mechanism can be a pneumatic, hydraulic, or electrical mechanism, or can utilize a controllable fluid which can be solidified by application of an electric current. The insert member can then be released for movement when the gas is injected by the use of various devices, such as position sensors, cavity pressure sensors, timers, or the like, or the deactivation of the electric current. The release or deactivation of the locking mechanism can be based or triggered in a number of different ways, such as by attainment of certain pressures in the mold, process timing sequences, linear movement of the injection screw, etc.
In another embodiment, the insert member is rotatable and has a recess or groove therein. Once the plastic packing step is completed, the insert member is rotated so that the recess or groove opens up into the mold cavity. The injection of the gas displaces a portion of the still-fluent plastic into the recess or groove, thereby forming the complete article.